For the collection and preparation of donor blood in the bloodbank and for the processing of medical fluids in the hospital, there is an increasing use of bloodbag systems and infusion sets made of flexible plastic, e.g. polyvinylchloride (PVC).
These bags often contain a fluid and are connected to each other by means of flexible plastic tubing. This makes them closed systems that do not lose internal sterility when the fluid is pressed from one bag to another. The tubing of the infusion sets as they are used for the infusion of medical fluids into a patient in the hospital are also generally made of flexible plastic, e.g. PVC.
In addition to advantages in costs, sterilization ability, nonbreakable and medical suitability, one of the major advantages for the use of such plastic systems is the ability to close the tubing hermetically. Methods for hermetically closing this type of tubing are e.g. the clamping of the tubing with a clamp or by means of a lead. However, an even more popular way of closing this tubing is by sealing.
For the sealing of medical plastic tubing, devices are already on the market, for example, those marketed by Sebra of Tucson, Ariz., U.S.A. Devices of this type contain an electrical circuit for the generation of energy and a pair of electrodes, the sealing jaws, between which the tubing to be sealed is introduced. The generated energy is conducted to the electrodes and causes a local heating of the tubing. When sufficient energy is delivered to the sealing jaws and when these jaws are pressed toward each other at the same time, a seal is formed in the tubing. In general, the generated energy is of a high frequency or radio frequency (RF) form, where the sealing jaws function as capacitor plates.
This technique is described inter alia in U.S. Pat. Nos. 4,013,860, 4,186,292, 4,390,832, 4,490,598, 4,529,832, 4,488,028, 4,491,711 and 4,496,819, all assigned to Engineering & Research Associates Inc., of Tucson, Ariz., U.S.A.
The devices described in these patents contain an RF generator that generates electrical energy with a fundamental frequency of 40.68 MHz, connected to a sealing head through a 50 Ohm coaxial cable. The sealing head contains a resonance circuit that transforms the applied energy to a high voltage. This high voltage, approximately 1500 volts, is necessary to generate enough heat in the PVC to make it melt.
In this sealing process, the plastic tubing is introduced between the two sealing jaws, the capacitor plates, and these jaws are pressed together. The tubing will now be heated when the RF field is generated between these jaws. This heating will make the tubing dent under the pressure of the two jaws and the jaws will approach each other. This will increase the RF field and subsequently increase the generated heat and dent the tubing even more, etc.
This avalanche effect melts the PVC tubing, thereby forming a seal. The process depends on the thickness of the tubing, the amount of RF energy and the force that clamps the jaws together. A normal sealing procedure will take about 1 or 2 seconds. The RF energy can now be stopped and a cooling time of approximately 1 second can be effective.
The heat is generated because the dipole of the PVC molecule vibrates in the RF field. As the vibration of the dipoles increases in intensity the more intensive the dipoles vibrate, the greater will be the generation of heat. There are, however, limitations. Strongly vibrating dipoles can fall apart decay into in charged particles that can collide with other dipoles, which will thereupon decay apart, etc. Thus a chain reaction will develop, generally called a breakdown, which will cause generation of so much heat, that the material will burn.
The vibration intensity has a direct connection with the power level of the electric field that is built by the sealing electrodes. To attain a good heat development, it turns out to be necessary to developed up a strong electrical field. The critical breakdown level should however never be reached. It is, therefore, of vital importance for good seal quality in a medical plastic articles that between the sealing electrodes a strong (exceeding the breakdown value) and particularly constant, electrical field is generated which, however, should not exceed the breakdown level.
A sealing apparatus for the sealing of medical plastics generally consists of two main parts, the energy generator and the sealing head, that is responsible for development of the electric field between the two sealing jaws. The sealing head, therefore, contains a resonance circuit or resonant network.
In general, to "resonate" means to "vibrate simultaneously". In this special case, we talk about an electric circuit that is able to oscillate in consonance with an external frequency. In normal mechanics, resonances are commonly known. A tuning fork for instance will only vibrate when a tone is offered that is equal or almost equal to the resonant frequency of this fork. It will not react to any other tone or frequency.
In electronics, one can build a similar resonant circuit with a capacitance (capacitor) and an inductance (coil). In such a circuit, a constant exchange of energy levels between the magnetic field of the coil and the electric field of the capacitor will take place. This circuit will in theory be in resonance when the apparent resistance of both components is equal, i.e. when: EQU 2.pi.fL=1/2.pi.fC
with
f=resonance frequency in Hertz PA0 L=self-induction in Henry PA0 C=capacity in Farad, PA0 L=self-induction in Henry PA0 C=capacity in Farad. PA0 C=capacitance of the capacitor PA0 A=surface of the capacitor plates PA0 .epsilon.=dielectric constant PA0 s=distance between the plates
As has been stated before, the sealing head contains such a resonant circuit. The voltage undergoes a resonance rise to a higher level, where the sealing electrodes serve as capacitors. From the description of the resonant circuit, it seems clear that it is important for a maximizing in the rise, that the circuit is tuned exactly to the frequency of the supplied signal.
This means that: ##EQU1## wherein: f=the resonance frequency of the circuit in Hertz
The self-inductance generally consists of a couple of coils, fixed on a support. The sealing electrodes serve as the capacitor, and when those components are designed correctly, this circuit will be in resonance with the frequency of the supplied electrical signal.
Different phases characterize the sealing process. The process starts by the formation of the electrical field which will heat the plastic. Due to the pressure applied to the electrodes, the tubing will dent and the distance between the electrodes will decrease. Depending on the dimensioning of the resonance circuit, this will now either be tuned or detuned. The sealing power will accordingly increase or decrease.
In the second case, the sealing process will prematurely end itself. The seal stays relatively thick and there is no actual melting of the plastic. The seal quality will be bad.
In the first case, an avalanche effect will take place that ends with a great heat development that will melt and seal the plastic. Thereafter the energy can be switched off and a cooling phase can start.
Thus the resonant circuit should be designed in such a way that the resonance point is not too early in the sealing process. However in certain circumstances, low temperature or stiff tubing, this resonance point will not even be reached within the set sealing time and there will be no sealing at all. Extension of the sealing time is not desired because of production time loss.
However a problem arises during the sealing process. Because the plastic heats up in the electric field between the sealing electrodes, these electrodes will move towards each other when pressure is applied to them. This will influence the capacitance of the capacitor. EQU C=A.epsilon./s
where
The capacitance of the capacitor increases with a decreasing distance of the plates. Moreover the dielectric constant of many types of material is strongly dependent on the temperature.
During the sealing process of PVC this constant increases six times for 27 MHz between 20.degree. C. (ambient temperature) and 140.degree. C. (melting temperature), so that the temperature rise alone can increase the capacitance of the capacitor six times.
Due to the increasing capacitance, the resonant circuit in the sealing head will detune itself during the sealing process, which will cause less resonance rise and less heating power.
Detuning of the resonance circuit can also be caused when a tubing filled with a fluid is introduced between the sealing electrodes. Most fluids are excellent conductors, thereby apparently decreasing the apparent distance of the capacitor plates. Other causes of detuning can be different materials that are to be sealed (with different dielectric constants) and aging of the components.
Thus the detuning of the resonance circuit during the sealing process or caused by other factors has an unfavorable influence on the sealing quality. It also has an unfavorable influence on the effect of possible electronic filters that are meant to suppress spurious radiation.
All these disadvantages indicate the need for a sealing head provided with a resonance circuit which will stay in resonance during the complete sealing process.